Sahih Muslim 1304
Sahih Muslim · Book 15: The Book of Pilgrimage · Shaving the head is preferrable to cutting the hair, although cutting the hair is permissible
وَحَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ ، حَدَّثَنَا يَعْقُوبُ، وَهُوَ ابْنُ عَبْدِ الرَّحْمَنِ الْقَارِيُّ ح وَحَدَّثَنَا قُتَيْبَةُ ، حَدَّثَنَا حَاتِمٌ، - يَعْنِي ابْنَ إِسْمَاعِيلَ - كِلاَهُمَا عَنْ مُوسَى بْنِ عُقْبَةَ ، عَنْ نَافِعٍ ، عَنِ ابْنِ عُمَرَ ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم حَلَقَ رَأْسَهُ فِي حَجَّةِ الْوَدَاعِ .
Ibn Umar reported that Allah's Messenger (ﷺ) got his head shaved on the occasion of the Farewell Pilgrimage.